Some patients who take dopamine-boosting drugs to alleviate Parkinson’s symptoms exhibit behavior more fitting for drug addicts. Learning why could help doctors better understand addiction.

Mark Stacy, associate professor of neurology at Duke University Medical Center, figured more than coincidence was at play when, during the course of two weeks in 1999, he discovered that two of his Parkinson’s patients had each lost $60,000 gambling after he increased their medication.

Then Joseph Friedman, clinical professor of neurology at Brown Medical School, reported the first case of punding—slang for the obsessive,
repetitive behaviors methamphetamine addicts exhibit—in a Parkinson’s patient. “He was an accountant and tallied figures over and over again,” says Friedman. “Another patient couldn’t stop trimming her hedges in winter, another obsessively pulled weeds and one couldn’t go grocery shopping because she couldn’t stop reading labels on cans.” Two of Friedman’s patients, both quadriplegic, became addicted to levadopa, a drug that replenishes dopamine in neurons that are failing to make enough. “They received no motor benefit from L-dopa, but they craved it like a drug addict.”

Compulsive behaviors—gambling, shopping and hypersexuality are the most common—are striking in people who would be expected to have what some describe as a “Parkinson’s personality,” characterized by
risk-averse, straitlaced behavior due to a gradual loss in dopamine production.
Though it’s not clear precisely what’s happening, the leading hypothesis is that Parkinson’s drugs that stimulate dopamine receptors
in the motor-system pathway are spilling into the pathway that activates reward and motivation. Reducing the medication alleviates the behavior but may cause significant motor difficulty.

Although impulse-control disorders affect a small number of Parkinson’s patients—Stacy has treated 5,000 patients and has seen only about 30 with the problem—it may provide one more clue to treating addiction. “Understanding how changing a medicine dose causes some Parkinson’s patients to exhibit these destructive behaviors could help us figure out how to block that change, which may lead to treatments for addiction,” says Stacy, who hopes to get funding to do fMRI studies of pathological gamblers and Parkinson’s patients.